To detect a groundspeed of a transport machine such as an automobile or a railroad vehicle, a method for obtaining a speed by measurement of the number of rotations of wheels is generally known. Such a method is known to fail to measure a groundspeed at a time of wheel slip, and to cause a measurement error when a wheel diameter is changed due to deflation of a tire or wear of a wheel.
A speed measuring device is also known in which the device calculates a groundspeed by using a radar module of a millimeter wave band or a micro wave band, emitting a continuous electromagnetic wave from the radar module, receiving a reflected wave of the electromagnetic wave to measure a frequency change quantity of the reflected wave (see, e.g., Patent Literature 1). Since the device uses a non-contact type method, a groundspeed can be measured even at a time of slip and change of a wheel diameter causes no influence.
Such radar module using an electromagnetic wave generally has wide directivity of an antenna. Thus, the directivity has to be sharpened by a lens. For example, Patent Literature 2 proposes a radar module that seals an MMIC (monolithic microwave integrated circuit) chip that has an active circuit, e.g., an oscillator or a mixer and an antenna mounted on a same semiconductor substrate in a resin package, and mounts a lens by bonding at a position above the antenna on a surface of the resin package. Patent Literature 3 proposes a configuration mounting a lens in contact with an opening of a tapered slot antenna. Patent Literature 4 proposes a configuration in which a dielectric oscillator is disposed near an end of a plane dielectric line to propagate an electromagnetic wave, an electromagnetic wave is emitted to air by a resonance phenomenon and a lens is arranged with a gap above the oscillator.